The present invention relates to a polyester film for membrane switch, and in detail, the present invention relates to a polyethylene naphthalate film for membrane switch, which is excellent in the transparency and dimensional stability at processing.
Furthermore in detail, the present invention relates to a biaxially oriented polyethylene naphthalate film for membrane switch, which shows the haze-increasing rate represented by the formula: ##EQU1## wherein H.sub.1 is the haze before subjecting the film to heat treatment and H.sub.2 is the haze after subjecting the film to heat treatment, of not more than 20% and both the heat-shrinkage in the longitudinal direction and the heat shrinkage in the transversal direction of not more than 0.5 % at subjecting the film to heat treatment for 2 hours at 150.degree. C.
Accompanying with the modernization of the electrical and electronic parts, the switches are shifting from the conventional push-type, slide-type or rotary-type to the thin membrane switch such as membrane switch, touch panel, transparent touch panel, etc., using a plastic film and particularly, in the card-type electronic calculator, the membrane switches have been used widely.
As the method for producing the membrane switch, (i) a method in which a circuit is printed on the surface of a polyethylene terephthalate film (hereinafter referred to as PET film) with an electroconductive paint, the spacer part is painted with an electroinsulating paint thereon and the thus painted films are piled and stuck together while facing the painted surfaces to each other, or (ii) a method in which a circuit is printed on the surface of PET film with an electroconductive paint, and the thus painted PET films are stuck together while facing the painted surfaces to each other and interposing an insulating material as the spacer between the two PET films, has hitherto been carried out.
As an electroconductive material for circuit of the membrane switch using the PET films, a silver series electroconductive material and a carbon series electroconductive material covering it are generally used.
In order to form an electroconductive circuit on the surface of the PET film, at first the silver paste is screen-printed on the PET film, and after drying and solidifying the paste, a carbon paste is further painted so as to cover the part printed by the silver paint. Namely, a method by which the screen-printing is carried out more than two times has been adopted.
Hereupon, although it is necessary for drying the painted paste after printing to subject it to heat treatment for from a few minute to one hour at a temperature in the range of from 130.degree. to 180.degree. C., under such drying condition, the PET film shrinks by heating and as a result of the dimensional change of PET film the film flatness of the PET film is spoiled, thereby making difficult to carry out the printing in less than a second with favorable accuracy. For instance, a dimensional difference is formed between the silver paste printing and the printed pattern of the carbon paste printed after drying the silver paste, thereby it becomes impossible to carry out the accurate printing. In such a situation, there occurs problems that the prevention of the migration due to the carbon paste and the decrease of the resistance value become insufficient.
Further, there occurs a problem that it becomes difficult to stick the spacer parts (printed surfaces) of the PET films together while facing the electroconductive circuit parts to each other.
Hitherto, in order to solve these problem, the PET film is subjected to heatset treatment as a sheet under the severer conditions than the drying conditions after printing, thereby preliminarily shrinking the film to restrict the dimensional change of the PET film during the drying step to the minimum.
However, in the case of carrying out the heatset treatment, the oligomer of polyethylene terephthalate educes on the PET film and sticks thereto, thereby significantly spoiling the appearance of the film, increasing the turbidity (film haze) of the film and decreasing the transparency of the film. In the case of using the film of which the transparency has been spoiled, for the membrane switch, the appearance of the switch is significantly damaged. Further, in the case of using the thus spoiled film as a base film of the membrane switch of the electronic calculator provided with a solar cell, etc., since a part of the film must remain in a transparent state in order to use such a transparent part as a protecting film for the solar cell, the capacity of the solar cell in the case where the film has haze is severely reduced.
Accordingly, in the membrane switch, it is extremely important to retain the transparency of the film and for such a purpose, it is necessary to prevent the eduction and sticking of the oligomer on the surface of the PET film. For the purpose, each of the sheet-form PET is wiped to remove the educed and sticking oligomer.
Furthermore, since the printing is carried out after subjecting the PET sheet to heatset treatment, a high processing cost needs and accordingly, a continuous printing has been also demanded.
As a result of the present inventor's studies in consideration of the above-mentioned circumstances, it has been found by the present inventor that the problems can be solved by using, instead of polyethylene terephthalate film, polyethylene naphthalate film showing a heat-shrinkage of less than the specified value and a haze-increasing rate before and after thermal treatment of less than the specified value, and on the basis of the finding, the present inventor has attained the present invention.